Photonic input/output port

ABSTRACT

The present I/O ports comprise (1) a layered structure comprising (a) an unpatterned superstrate having at least one layer, (b) an unpatterned substrate having at least one layer, and (c) at least one intermediate layer sandwiched between the unpatterned superstrate and the unpatterned substrate, (2) a coupling region that is within the at least one intermediate layer and that comprises an arrangement of at least one optical scattering element and (3) at least one output waveguide. The present I/O ports can be effectively used in balanced photonic circuits and unbalanced photonic circuits.

Cross-Reference To Related Applications

[0001] This application relates to and claims priority benefits fromU.S. Provisional Patent Application Serial No. 60/281,650, filed Apr. 5,2001, which is incorporated by reference herein in its entirety, fromU.S. Provisional Patent Application Serial No. 60/302,256, filed Jun.29, 2001, which is incorporated by reference herein in its entirety andfrom U.S. Provisional Patent Application Serial No. 60/332,339, filedNov. 21, 2001, which is incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

[0002] This invention is related to the field of integrated optics (thatis, integrated photonics). In particular, the present devices arephotonic input/output (I/O) ports designed for effective coupling ofoptical signals from a planar photonic circuit to an external opticalfiber, or vice versa. Moreover, the present I/O ports enable theimplementation of balanced photonic circuits for polarizationindependent operation.

BACKGROUND OF THE INVENTION

[0003] Effective optical (that is, photonic) telecommunication systemsrequire high-performance, low-cost photonic devices. Such a requirementhas motivated development of integrated photonic circuits that areplanar or substantially planar. Those circuits interface with otherdevices and system components using input/output (I/O) ports, which maybe referred to as couplers or grating couplers and which typicallyoptically connect planar or substantially planar circuits to cylindricaloptical fibers. Such I/O ports can act as input ports, output ports, orbi-directional ports. As used herein, the terms input port(s), outputport(s), bi-directional port(s) and I/O port(s) may be usedinterchangeably. In other words, unless otherwise specified, each ofthose terms contemplates and includes all of those terms.

[0004] In planar or substantially planar photonic circuits, coupling toor from an optical fiber is commonly achieved in an axial geometryarrangement using a system of lenses (FIG. 1(a)), or by directlyattaching the fiber to the planar or substantially planar photoniccircuit (FIG. 1(b)). The problems with such approaches include the needfor photonic circuit surfaces of high quality (that is, highly smooth,planar or substantially planar surfaces, which may be prepared bycleaving and/or polishing and through which a photonic signal may pass),and the need for highly accurate mechanical alignment of micro-photonicelements. In addition, to be effective, many planar or substantiallyplanar photonic circuits are required to be polarization independent(that is, to operate substantially the same way for any inputpolarization). Consistently achieving polarization independence ineffective axially-coupled planar or substantially planar circuits hasproven to be generally difficult and, in some cases, has resulted in I/Oports that compromise a circuit's overall performance or flexibility.Moreover, devices, such as I/O ports, fabricated on the same wafercannot be properly tested until after separation into individualelements. Such testing constraints have further complicated efforts tocommercialize effective telecommunication systems.

[0005] The present I/O ports can be effectively incorporated into planaror substantially planar photonic circuits, and the present I/O portseffectively couple light to optical fibers. The present I/O ports caneffectively couple light at normal or near-normal incidence to the planeof the photonic circuit. As used herein, the term “near-normal” shallmean and include angles up to approximately 300 away from normal (thatis angles ranging from approximately −30° to approximately +30°), andthe term “off-normal” shall mean and include all “near-normal” anglesexcept those angles equal to approximately 020 . In near normalgeometry, light from an optical fiber is shone either indirectly, usinga system of lenses (as shown in FIG. 2(a)), or directly (as shown inFIG. 2(b)) onto the input port located on the top (or bottom) surface ofthe planar or substantially planar photonic circuit.

[0006] In effective optical telecommunication systems, particularlythose employing dense wavelength-division multiplexing (DWDM), I/O portsare usually operable over a wide band of input frequencies and, thus,over a wide band of input wavelengths. Current commercially availableoptical telecommunications systems employ wavelengths from approximately1525 to approximately 1565 nm, a range known as the C-band, andwavelengths from approximately 1565 to approximately 1620 nm, a rangeknown as the L-band. It is therefore important to control (for example,to maximize), the operational bandwidth of an I/O port. As used herein,the term “control” shall mean and include minimize, maximize, reduce,increase and/or achieve a desired or effective level or range, unlessotherwise specified.

[0007] It is also important to control (for example, to maximize)coupling efficiency, with coupling efficiency being the fraction oflight incident upon the I/O port that is transferred into the coupledcircuit. Similarly, controlled insertion loss is desired. Insertionloss, expressed in decibels (dB), is defined as ten times the base tenlogarithm of the inverse of the coupling efficiency.

[0008] Prior work in connection with or relating to I/O ports featuringthe geometry of FIG. 2 has been conducted. Such prior I/O ports havebeen used in connection with normal and near normal incidence couplingand typically comprise an optical waveguide and one-dimensional orapproximately one-dimensional grating, which is a periodic arrangementof grooves or straight lines. The grating grooves or lines serve asoptical scattering elements for incident light, and are arranged todirect near-normal incident light into the plane of the device in acoupling region.

[0009] Prior work in connection with or relating to I/O ports featuringthe geometry of FIG. 2 can be distinguished from the present I/O portsby, for example, considering the index contrast in a coupling region,Δn. Δn is defined as the difference between the maximum refractive indexand the minimum refractive index of the respective constituent materialsin the coupling region (that is, the respective constituent materialscomprising an optical scattering element, which is defined below). Thoseconstituent materials may, as explained below, be air and the materialof which the coupling region is made. Prior work onnear-normal-incidence couplers has concerned low index contrast gratingsin low index contrast waveguides. Such prior work has suffered fromlimited coupling bandwidth, insertion loss and/or sensitivity to angularmisalignment.

[0010] It is desirable to achieve effective operation of a planar orsubstantially planar photonic circuit with the simple direct fiberattachment of FIG. 2(b). For a conventional single-mode optical fiber(that is, an optical fiber that supports only one propagating mode atthe operating wavelength), such as the fiber illustrated in FIG. 2(b),the spatial profile of the optical mode can be considered Gaussian. Themode field diameter of the optical mode, which diameter is defined asthe full width at the 1/e² intensity points, is typically on the orderof 10 μm. Prior planar or substantially planar photonic circuits sufferfrom higher insertion loss with such small mode field diameters, and,accordingly, such circuits usually require beam expanding optics inorder to adapt the mode field diameter of the fiber to the larger modefield diameter characteristic of prior I/O ports.

[0011] Prior I/O ports designed to couple light at near-normal incidencetypically suffer from excessive polarization dependence. In other words,certain optical performance specifications for a photonic circuit (suchas, for example, the insertion loss for the circuit), depend upon thepolarization of the input light. More specifically, prior work showsthat prior near-normal incidence I/O ports suffer frompolarization-dependent loss (PDL), which is defined as the maximumamount of insertion loss variation observed for the photonic circuitwhile varying the input light over all possible states of polarization.

[0012] A number of suggested solutions to the above problems, whichsuggested solutions are taught by prior work, involve complex systems ofmicro-optical elements or complex fabrication sequences. Such suggestedsolutions are unacceptable for one or more reasons, including, thetendency for such micro-optical elements to move (physically) over time,the cost of assembling those complex systems and the lower yieldstypically attributable to complex fabrication sequences.

[0013] Another important aspect of the design of an effective I/O portis its compatibility with in-plane waveguide interconnects of the planarphotonic integrated circuit. It is preferable to use single modewaveguides in planar photonic integrated circuits. In many cases, thetypical transverse mode profile of guided light is on the order of 1 μm.As mentioned above, the typical mode field diameter exiting from asingle mode optical fiber is on the order of 10 μm. Therefore, toachieve meaningful effectiveness in an I/O port, an additional mode-sizeconverting optical element is needed. One such appropriate element is aplanar waveguide lens.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic diagram showing conventional axial geometryfor coupling light into a planar photonic waveguide by (a) a system oflenses, and (b) by direct fiber attachment.

[0015]FIG. 2 is a schematic diagram showing normal-incidence couplingwith (a) a system of lenses, and (b) direct fiber coupling.

[0016]FIG. 3 is a plan-view schematic diagram of I/O port designssuitable for operation with light of a known, fixed input polarization(illustrated in (a) and (b)) and with light of an unknown or varyinginput polarization (illustrated in (c) and (d)).

[0017]FIG. 4 shows an illustrative view of a portion of an I/O portcorresponding to FIG. 3(a).

[0018]FIG. 5 is a schematic diagram showing (a) a preferred embodimenthigh index contrast I/O port, and (b) a conventional low index contrastI/O port.

[0019]FIG. 6 is a plot showing first order scattering amplitudes as afunction of wavelength. The two curves denoted by A and B show resultsfor the I/O ports of FIGS. 5(a) and (b), respectively.

[0020]FIG. 7 is a plot showing full width half maximum (FWHM) couplingbandwidth as a function of optical scattering element depth. The twocurves denoted by A and B show results for the I/O ports of FIGS. 5(a)and (b), respectively.

[0021]FIG. 8 is a plot showing FWHM coupling bandwidth as a function ofthe substrate layer (16) thickness, t₁, for the I/O port of FIG. 5(a).

[0022]FIG. 9 is a plot showing branching ratio as a function of thesubstrate layer (16) thickness, t₁, for the I/O port of FIG. 5(a).

[0023]FIG. 10 is a plot showing variation in the field loss parameter asa function of the substrate layer (16) thickness, t₁, for the I/O portof FIG. 5(a).

[0024]FIG. 11 is a plan view schematic of the coupling region of apreferred embodiment I/O port suitable for use with light of unknown orvarying input polarization.

[0025]FIG. 12 shows the Gaussian intensity profile (dashed line) andcorresponding power loss parameter (solid line) for a preferredembodiment I/O port.

[0026]FIG. 13 shows the pitch (“+” symbols) and radius (“x” symbols) asa function of position within the coupling region for a preferredembodiment I/O port.

[0027]FIG. 14 shows a cross-sectional view of a preferred embodiment I/Oport fabricated with silicon-on-insulator (SOI) materials. In FIG.14(a), optical scattering elements are patterned in an upper siliconlayer. Subsequently, an unpatterned SiO₂ layer is attached (for example,by wafer bonding).

[0028]FIG. 15(a) is a schematic diagram showing a photonic circuit inwhich an input port has two outputs of identical polarization, whichoutputs are coupled to two substantially identical optical elements (forexample, optical filters), the outputs of which are directed to anoutput port. FIG. 15(a) also shows the use of intermediate waveguides.FIG. 15(b) is a schematic diagram showing a device where two outputsfrom an input port are coupled into two potentially different opticalelements, a two-input combiner, such as a multi-mode coupler, and into asingle output port. FIG. 15(b) also shows the use of intermediatewaveguides.

[0029]FIG. 16(a) is a schematic diagram showing unpatterned superstratelayer(s), unpatterned substrate layer(s) and intermediate layer(s),which are not yet patterned, of the present I/O ports. FIG. 16(b) is aschematic diagram showing the same layers as FIG. 16(a) plus a singleoptical scattering element (namely, a cylindrical hole) in theintermediate layer(s).

SUMMARY OF THE INVENTION

[0030] The present I/O ports, which are suitable for incorporation intoa photonic circuit, comprise (1) a layered structure comprising (a) anunpatterned superstrate having at least one layer, (b) an unpatternedsubstrate having at least one layer and (c) at least one intermediatelayer sandwiched between the unpatterned superstrate and the unpatternedsubstrate, (2) a coupling region that is within the at least oneintermediate layer and that comprises an arrangement of at least oneoptical scattering element and (3) at least one output waveguide todirect (that is, propagate) output light from the coupling region toanother part of the photonic circuit. The present I/O ports can beeffectively used in balanced photonic circuits and unbalanced photoniccircuits.

[0031] The present I/O ports solve a number of the problems associatedwith coupling light from an optical fiber into a planar photoniccircuit. Compared to prior I/O ports, the present I/O ports operate moreeffectively at higher coupling efficiency and over a broader bandwidththan any previously implemented, otherwise comparable I/O ports. Thepresent I/O ports are capable of coupling light that enters the I/Oports at normal, near-normal and off-normal incidence (that is, withinapproximately 30° of the normal of the top (or bottom) plane of thephotonic circuit), and are particularly effective at coupling light thatenters the I/O ports at approximately 12° from the normal of the top (orbottom) plane of the photonic circuit. The present I/O ports are formedusing an arrangement of higher refractive index contrast opticalscattering elements. Several objects and advantages of the present I/Oports include:

[0032] a) effectively coupling a range of optical wavelengths;

[0033] b) coupling an incident or outgoing beam at a predetermined angle(for example, normal or near normal) to the surface of the coupler;

[0034] c) controlling (for example, maximizing) the in- and out-couplingof a Gaussian-like intensity profile, with a mode field diameter ofapproximately 10 μm or less; and

[0035] d) controlling (for example, reducing) the insertion loss causedby, for example, a mismatch of effective indices across the outputboundary(-ies) of the I/O port;

[0036] e) controlling (for example, reducing) the polarizationdependence of I/O port characteristics such as insertion loss andtransmission delay, which is the time delay incurred by an opticalsignal travelling through an I/O port.

[0037] Further objects and advantages of the present I/O ports willbecome apparent from a consideration of the accompanying drawings anddescription.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

[0038] The present I/O ports comprise a planar or substantially planarlayered structure comprising an unpatterned superstrate having at leastone layer, an unpatterned substrate having at least one layer and atleast one intermediate layer, which is sandwiched between theunpatterned superstrate and the unpatterned substrate, a coupling regionthat is within the at least one intermediate layer and that comprises apattern of at least one optical scattering element and at least oneoutput waveguide to direct output light from the coupling region toanother part of the photonic circuit. In operation, the coupling regionis illuminated by external optical input (for example, input light froma fiber). From the coupling region, the light is directed (that is,propagated) to the at least one output waveguide, and, in someembodiments of the present I/O ports, the directing of that light iseffected by at least one output region, which, like the rest of theelements comprising the present I/O ports, subsists in the layeredstructure. The at least one output region is specifically locatedbetween the coupling region and the at least one output waveguide. Theat least one output region can also adapt the optical mode of the lightthat is exiting the coupling region and propagating across at least oneoutput boundary to the optical mode of the light that is entering the atleast one output waveguide.

[0039] Representative arrangements of a coupling region, at least oneoutput region, and at least one output waveguide are illustrated in FIG.3, which shows a number of I/O port arrangements in plan view. In FIG.3, x and y represent directions in the plane of the structure, and inputlight is directed into the coupling region in the −z direction, orwithin approximately 30° of the −z direction, for normal or near-normalincidence, respectively. Alternatively, input light is directed into thecoupling region from below the plane, in the +z direction, or withinapproximately 30° of the +z direction, for normal or near-normalincidence, respectively.

[0040] FIGS. 3(a) and (b) show I/O ports that feature a layeredstructure and are suitable for directing input light of a knownpolarization. FIG. 3(b) shows an arrangement especially suitable forlight at normal incidence. In those I/O ports, light tends to exit eachrespective coupling region (1) in both the +x and −x directions, and, inFIG. 3(b), an in-plane reflector region (2), which also subsists in thelayered structure and is located behind coupling region (1), redirectslight exiting the I/O ports in the −x direction towards the outputregion (3) and thus towards the output waveguide (4). Each of the I/Oports of FIGS. 3(a) and (b) further comprises an output boundary (5).The output boundary (5) is defined to be the area of demarcation betweenthe coupling region (1) and the output region (3). FIGS. 3(c) and (d)show I/O ports suitable for input light of an unknown or varyingpolarization. In those I/O ports, light is ultimately directed towards afirst output waveguide (6), a second output waveguide (7), or bothoutput waveguides (6, 7) depending upon the light's polarization. FIG.3(c) shows a single output region (8), with light exiting couplingregion (9) and crossing a single output boundary (10). FIG. 3(d) showsan I/O port that is preferred for directing light into first outputregion (11) and second output region (12), and then into first andsecond output waveguides (6, 7), respectively, which are arranged to beapproximately orthogonal or orthogonal to one another within a plane.The I/O port shown in FIG. 3(d) further comprises first output boundary(13) and second output boundary (14), respectively. By analogy to FIG.3(b), the arrangements in FIGS. 3(c) and (d), which are especiallysuitable for use with light at off-normal incidence, can be adapted forinput at normal incidence by inclusion of one or two in-plane reflectorregions, respectively. An in-plane reflector region comprises in-planereflecting elements and, depending on the overall design of an I/O port,an in-plane reflector region may comprise more than one in-planereflecting element. Suitable in-plane reflecting elements includetwo-dimensional planar photonic crystal elements and high index contrastetched reflectors.

[0041] The present I/O ports may be fabricated as a layered structure bysuch patterning techniques as optical lithography, etching anddeposition. For example and as shown in FIG. 16(a), the layeredstructure may comprise one or more superstrate layers (56), one or moresubstrate layers (57), and one or more intermediate layers (58)sandwiched between the one or more superstrate layers (56) and the oneor more substrate layers (57). The present I/O ports feature appropriatearrangements of appropriate optical scattering elements that areincorporated into at least one intermediate layer. As explained above,in the present I/O ports, a coupling region lies within the at least oneintermediate layer. Notably, the output boundary, which was definedabove, is further characterized by the absence of optical scatteringelements on the outer edge of the output boundary, with the outer edgeof the output boundary defined to be the portion of the output boundarythat is adjacent to or near the output region. Herein, a layeredstructure can be further characterized by the properties of itsconstituent layers prior to any patterning. Those properties include therespective thicknesses and material compositions of the layers.Particular considerations relating to the sequence of fabrication stepsthat may be useful in fabricating the present I/O ports are describedlater in this specification.

[0042] As stated above, appropriate optical scattering elements can beappropriately arranged within the layered structures of the present I/Oports. An optical scattering element comprises a scattering material,material surrounding the scattering material and an interface betweenthe scattering material and the material surrounding the scatteringmaterial. The effect of the interface is that the index of refraction isnot constant in the x-y plane. In other words, the index of refractionfor the scattering material is different than the index of refractionfor the material. By way of example, and as shown in FIG. 16(b), anappropriately selected optical scattering element (59) may be acylindrical hole comprising air as a scattering material (60), material(61) surrounding (that is, forming a boundary around) the scatteringmaterial (60) and an interface (62) between the scattering material (60)and the surrounding material (61). The volume of a scattering element isactually the volume of the scattering element's scattering material(60). The intermediate layer(s) of the present I/O ports comprise one ormore materials, such as the material (61). An appropriate material foran intermediate layer may be GaAs (that is, gallium arsenide). As shownin FIG. 16(b), an example of an optical scattering element is acylindrical hole within a layered structure. Optical scattering elementsof various geometries (and various volumes) can be incorporated into thepresent I/O ports, depending upon the application. Another example of anoptical scattering element is a rectangular trench.

[0043] One feature of the present I/O ports that can further distinguishthem from prior I/O ports is that the index contrast in the couplingregion (that is, the index contrast of the optical scattering elements),Δn, is relatively high, preferably greater than or equal toapproximately 1. Prior work has concerned lower index contrast opticalscattering elements, which can result in such problems as anunacceptable restriction on the operating bandwidth of the I/O port,unacceptable restriction on the coupling efficiency of the I/O port,relatively high sensitivity to angular misalignment of the input lightwith respect to the I/O port, or a combination of some or all of thoseproblems.

[0044] There are several preferred arrangements of optical scatteringelements within the coupling regions of the present I/O ports. Some ofthose preferred arrangements are suitable for use with input light of aknown, fixed polarization. I/O ports that feature those preferredarrangements would be useful, for example, with polarization-preservingfiber inputs. In one preferred embodiment of the present I/O ports,which embodiment is depicted in FIG. 3(a), light is incident upon thecoupling region as a result of suitable positioning of the input fiber.The propagation of light through the I/O port embodied in FIG. 3(a) isdepicted in FIG. 4. As shown in FIG. 4, light is incident upon thecoupling region (1) at an angle θ with respect to the normal. That lightmay be polarized in either the “s” or “p” direction, as indicated inFIG. 4, or in some combination of those two directions. The opticaloutput from the coupling region (1) propagates across the outputboundary (5) and into the output region (3) at an angle φ with respectto the input beam. In the case of off-normal incidence, a forward I/Oport is defined as one in which light emerges from the coupling region(1) in a direction where φ>90°. Conversely, a backward I/O port, in thecase of off-normal incidence, is defined as one in which light emergesfrom the coupling region (1) in a direction where φ<90°. Off-normalincidence is usually the preferred configuration for a planar orsubstantially planar photonic circuit because off-normal incidencefacilitates achieving optical output from the coupling region in asubstantially single direction. Achieving effective optical output fromthe coupling region in a substantially single direction is dependentupon appropriate arrangement of appropriate optical scattering elementswithin the coupling region. Compared to operation at off normalincidence, operation at normal or near normal incidence can make itrelatively difficult to achieve output from the coupling region in asubstantially single direction. In fact, in order to achieve such outputwhile operating at normal incidence, one or more in-plane reflectorregions (2) as illustrated in FIG. 3(b), often have to be incorporatedinto the photonic circuit. An in-plane reflector region (2), such asillustrated in FIG. 3(b), and the associated traversal of the couplingregion by light redirected by the reflector, can result in a decrease ineffective coupling, or equivalently, increased insertion loss. A furtheradvantage of operating at off-normal incidence is that any reflectionfrom the surface of the coupler will not be directed back toward theinput, thereby reducing back reflection from the I/O port (with backreflection from the I/O port being defined as reflection of input lightfrom the I/O port back into the input fiber). For compatibility withstandardized angle-polished fibers, the operating angle θ can properlybe chosen to be approximately 12°. Taking into account refraction oflight at the end of a fiber, such an operating angle is appropriate forconventional angle-polished fibers, which are polished at an angle ofapproximately 8° from the longitudinal axis of the fiber. Where anoptical fiber is directly attached to an input/output port, the angle θcan properly be chosen to be approximately 8°.

[0045] Analysis of the preferred embodiments described below is achievedby one of several analysis techniques well known to one skilled in theart. We employ a self-consistent Green's function technique thataccurately describes the optical properties of arbitrarily thick, highdielectric contrast gratings in the planar waveguide geometry (See Cowanet al, “Resonant scattering and mode coupling in 2D textured planarwaveguides,” J. Opt. Soc. Amer. A18, (5), pp. 1160-1170, May 2001.) TheGreen's function technique is useful, in conjunction with coupled-waveequations, for approximating the behavior of the present I/O ports.Finite-difference time-domain (FDTD) is another technique that is usefulin quantitatively accounting for the effects of input mode fielddiameter and nonuniform arrangements of optical scattering elements.

[0046]FIG. 5(a) illustrates a preferred embodiment of a coupling region(1) for a present I/O port operating with light at a singlepolarization. In FIG. 5(a), optical scattering elements (15) (that is,rectangular trenches) are in a uniform (in particular, and by way ofexample, a symmetrical) arrangement, with each optical scatteringelement (15) having a depth, d₁, of approximately 180 nm, Λa pitch, Λ,of approximately 860 nm, and a width, b, defined by a duty cycle, b/Λ,of approximately 0.3. A uniform arrangement of optical scatteringelements is an arrangement of optical scattering elements wherein eachof the optical scattering elements has the same or approximately thesame volume and is evenly or approximately evenly spaced from eachneighboring optical scattering element. Underneath the opticalscattering elements, the substrate comprises an unpatterned substratelayer (16), in this case and for example, comprising Al_(x)O_(y), thathas a thickness, t₁, of approximately 1150 nm and a GaAs layer (17),which is located below the substrate layer (16) and serves as partialreflector beneath the coupling region and also as a physical support forthe I/O port. We take the refractive indices of GaAs and Al_(x)O_(y) tobe approximately 3.35 and 1.60, respectively.

[0047] For comparison, FIG. 5(b) shows a comparable coupling region fora prior I/O port, with that coupling region comprising low indexcontrast optical scattering elements. The structure of that couplingregion, with optical scattering elements (18) and substrate layers (19,20) is similar to the structure described in FIG. 10 of “Analysis anddesign of grating couplers,” T. Tamir and A. T. Peng, Appl. Phys. 14,pp. 235-254 (1977). In this example of prior work, the refractive indexof Material 1 and the refractive index of Material 2 are taken to beapproximately 1.732 and 1.517, respectively. In FIG. 5(b), each opticalscattering element (18) has a depth, d₁, of approximately 180 nm andmaterial (19) has a layer thickness, t₁, of approximately 500 nm. Oneadvantage that the present I/O ports have over such prior work isillustrated in FIG. 6 for incidence at an angle of θ=10°, which anglecorresponds to a forward I/O port. As illustrated in FIG. 6, input lightis polarized with its electric field along the length of the bars (thatis, s-polarized as indicated in FIG. 4) and the output light from thecoupling region is polarized with its electric field in the plane of thelayers. FIG. 6 shows the square of the magnitude of the first orderscattering amplitude as a function of wavelength, where curve A is forthe high index contrast I/O port of FIG. 5(a) and curve B is for theconventional I/O port (that is, conventional grating coupler) of FIG.5(b). Note that those curves correspond to structures where each opticalscattering element has the same depth of approximately 180 nm. Sincecoupling efficiency is proportional to the square of the scatteringamplitude, the widths of the peaks shown are indicative of theoperational bandwidth of the I/O ports. The FWHM of the couplingamplitude of a conventional I/O port is approximately 2 nm, while theFWHM for the preferred embodiment discussed above is approximately 50nm. This improvement, by a factor of 25, is due to the strong opticalscattering achieved with high index contrast optical scatteringelements.

[0048] As stated above, it is also important to control the operational(that is, the effective coupling) bandwidth of an I/O port. One approachthat may be used to increase the operational bandwidth of either of theI/O ports set forth in FIG. 5 is to increase the volume of the opticalscattering elements (by, for example, increasing the depth of theoptical scattering elements). However, because of practical limitationsto current etching techniques that approach has not produced generallyacceptable results. In addition, for conventional I/O ports, it is wellknown that the operational coupling bandwidth saturates when the depthsof the optical scattering elements increase. FIG. 7 shows the FWHMcoupling bandwidth determined from the scattering amplitude as afunction of the optical scattering element depths, d₁, for the two I/Oports of FIG. 5. FIG. 7 illustrates another advantage of the present I/Oports, namely, control over the operational bandwidth of an I/O port. Inparticular, curve B of FIG. 7 shows that the bandwidth of a conventionalI/O port saturates at a maximum value of approximately 4 nm, whereascurve A shows that a much larger (and, as such, much more controllable)operational bandwidth can be achieved with the present I/O ports. Infact, a coupling bandwidth greater than 100 nm can be achieved for highdielectric contrast structures that feature optical scattering elementswith depths larger than approximately 350 nm.

[0049] The present I/O ports provide additional design flexibility andadvantages over prior I/O ports. More specifically, the present I/Oports achieve more effective control over the operational bandwidth of aphotonic circuit because the present I/O ports comprise a layeredstructure with layers of appropriately selected thicknesses. Forexample, FIG. 8 shows the FWHM coupling bandwidth for the I/O port ofFIG. 5(a) as a function of the substrate layer (16) thickness, t₁. Byappropriate selection of the substrate layer (16) thickness, t₁, theFWHM coupling bandwidth may be further increased (by a factor of two inthe example of FIG. 8).

[0050] Another point to recognize is that, as a guided mode of lightpropagating in the coupling region interacts with optical scatteringelements, some of the light is lost (that is, radiated away towards thesuperstrate or the substrate). The branching ratio (BR), defined as thefraction of the total input light that radiates in the direction ofinterest for coupling, quantifies the magnitude of such a loss. An idealI/O port will have a branching ratio of unity. In the present I/O ports,the branching ratio may, as indicated above, be controlled byappropriately selecting layer thickness(es) for the superstrate, thesubstrate or both. FIG. 9 shows the branching ratio as a function of thesubstrate layer (16) thickness, t₁, for the I/O port of FIG. 5(a).Because a large BR is desirable for maximizing overall couplingefficiency of an I/O port, the substrate layer (16) thickness, t₁, maybe employed to optimize this parameter as well. Examination of FIGS. 8and 9 shows that by appropriately selecting the thickness of only asingle layer, the coupling bandwidth and the branching ratio are bothaffected simultaneously. However, for a given layered structure, thebranching ratio is not sensitive to commensurate changes in the pitchand duty cycle of the arranged optical scattering elements, while thecoupling bandwidth is highly sensitive to such changes. The calculationsin FIGS. 8 and 9 show that for the structure of FIG. 5(a) withΛ=approximately 860 nm and b/Λ=approximately 0.3, the branching ratio isapproximately 0.68 and the coupling bandwidth is approximately 46 nm.Similar calculations for a structure with Λ=approximately 775 nm andb/Λ=approximately 0.2, show that the branching ratio is the same,approximately 0.68, but the coupling bandwidth decreases toapproximately 18 nm. Thus, by coordinating the design of the substrateand superstrate layers with the design of the coupling region comprisingat least one optical scattering element, the overall coupling efficiencyof the I/O port and the coupling bandwidth may be controlledindependently.

[0051] Also, the larger coupling bandwidth associated with a higherindex contrast I/O port results in reduction of the angle sensitivity ofthe I/O port. For example, in an I/O port with a uniform arrangement ofotherwise appropriate optical scattering elements, the acceptance angleof the I/O port is related to the bandwidth by Δθ≈Δλ/(Λcos θ), where θis the optimum coupling angle and Λ is the optical scattering elementpitch. Thus, for reasons the same as or similar to those given above, bycoordinating the design of the superstrate layer(s) and substratelayer(s)with the design of the coupling region that is within theintermediate layer(s), the angle sensitivity of the I/O port may also becontrolled.

[0052] Compared to prior work, the present I/O ports significantlyreduce the input mode field diameter for effective coupling, therebyfacilitating direct connection of the I/O ports to conventional opticalfibers. Indeed, such facilitated direct connections reduce, if noteliminate, the need for intermediate optical elements, such as systemsof lenses, between the I/O ports and fibers.

[0053] It is well known that, for I/O ports with a uniform arrangementof optical scattering elements, there exists a mode field diameter thathas maximum coupling efficiency, and that this mode field diameter isinversely related to the power loss parameter, α_(p), that characterizesthe coupling region. Another parameter typically used to describe I/Oports is the field loss parameter, α=α_(p)/2. The typical mode fielddiameter of a beam exiting an optical fiber is approximately 10 μm and,therefore, in this case, the optimum value for the field loss parameteris α=approximately 1330 cm−¹. FIG. 10 shows the variation in field lossparameter for the high dielectric contrast I/O port of FIG. 5(a), withΛ=775 nm and b/Λ=0.2, as a function of the substrate layer thickness(16). As shown in FIG. 10, the appropriate field loss parameter for anapproximately 10 μm mode field diameter can be obtained by appropriateselection of the substrate layer (16) thickness, t₁. Again, coordinatingthe design of the superstrate layer(s) and substrate layer(s) with thedesign of the coupling region that is within the intermediate layer(s)allows one to effectively control the field loss parameter and, thus,the optimum coupling mode field diameter, independent of the bandwidthof the I/O port.

[0054] We now turn to preferred embodiments of the present I/O portsthat are suitable for effective coupling of input light of unknown orvarying polarization. Under such circumstances, one generally seeks I/Oport performance that does not significantly vary with any change ininput polarization. Common parameters to measure such variance (or lackthereof) include PDL, which measures the maximum variation of theinsertion loss as the input light varies over all possible polarizationstates. In addition (or alternatively), polarization mode dispersion(PMD) may be used to measure any variation in delay experienced by aninput signal. PMD specifically measures the maximum variation in delayfor input light passing through an I/O port, and that variation ismeasured over all possible polarization states.

[0055]FIG. 3(c) illustrates a preferred embodiment I/O port. For reasonsthe same as or similar to those set forth above, in the preferredembodiment of FIG. 3(b), off-normal incidence simplifies the design andimproves the performance of the I/O port such that the I/O porteffectively couples input light of unknown or varying polarization. Asexplained above, suitable ranges of angles of incidence for input lightare approximately −30°≦θ≦approximately −1° and approximately1°≦θ≦approximately 300 (that is, off-normal incidence).

[0056]FIG. 11 depicts a coupling region (21) for a forward I/O port,with θ=approximately 12°. In FIG. 11, the optical scattering elementsare cylindrical holes (22) that are in a uniform arrangement(specifically, and for example, the cylindrical holes (22) are in asquare lattice and have a pitch, a, of approximately 740 nm). The axesof the square lattice are oriented at 45° to the x and y axes. Lightfrom the coupling region (21) crosses an output boundary (23) in the +xdirection, which is the M symmetry direction of the square lattice. Thelight propagates away from the output boundary (23) as first output beam(24), second output beam (25) or both first and second output beams (24,25). Although not shown in FIG. 11, the layered structure of an I/O port(with the illustrated coupling region) may comprise a first intermediatelayer of GaAs, with a thickness of approximately 180 nm, and a secondintermediate layer of Al_(x)O_(y), with a thickness of approximately 300nm. The substrate comprises an upper substrate layer of Al_(x)O_(y),with a thickness of approximately 850 nm, and a lower substrate layer ofGaAs. The optical scattering elements extend through the intermediatelayers of GaAs and Al_(x)O_(y). Each of the cylindrical opticalscattering elements has a radius of approximately 188 nm. In thisembodiment of the present I/O ports, the power loss parameter for anyinput polarization is approximately 2000 cm⁻¹, thereby effectivelymatching an optical mode field diameter of approximately 14 μm over arange of frequencies within the C-band, while simultaneously achievinglow polarization dependent loss.

[0057] Having described the above features of the present I/O ports incontrolled polarization and varying or unknown polarizationapplications, it is now appropriate to consider additional features thatcan be incorporated into the present I/O ports.

[0058] There are several types of light loss (including scattering oflight out of the desired optical path and general dissipation of lightas it propagates). Of course, any such loss interferes with theeffectiveness of an I/O port.

[0059] A more specific type of loss occurs as a result of reflection oflight when light emerges from the coupling region and crosses an outputboundary. Such reflection can occur if there is a mismatch between theguided mode of light in the coupling region and the guided mode of lightin the output region. That reflection is due, in part, to the differencebetween the effective refractive index for the coupling region and theeffective refractive index(-ices) for the at least one output region.The effective refractive index, n_(e), is defined by the ratio of thespatial frequency in the material (or wavevector, k) to the angularoptical frequency of oscillation, ω, times the speed of light in vacuum,c, that is, n_(e) =kc/ω). Mode profile mismatch, which is defined to bea difference between two modes' spatial distribution of opticalintensity, can also contribute to loss of light (with loss of lightbeing reduced transmission of light into the output region). In priorlower-index-contrast I/O ports (or grating couplers), the amount of lossin crossing the output boundary was relatively small. However, theamount of that loss can be significant in I/O ports comprisinghigh-index-contrast optical scattering elements.

[0060] To eliminate a mismatch between the respective refractiveindices, mode matched material may be employed in the output region ofthe planar photonic crystal. For example, by etching a two-dimensionalarrangement of holes into an initially unpatterned layer structure, aplanar photonic crystal waveguide may be fabricated adjacent to a highdielectric contrast I/O port. By selecting appropriate pitch andsymmetry for that arrangement, and selecting appropriate volume(s) forthose holes, the effective refractive index of the planar photoniccrystal waveguide may be engineered to be approximately equal or equalto the effective refractive index of the high dielectric contrast I/Oport at a given wavelength. By selecting the lattice constant of theplanar photonic crystal material to be less than λ/(2n_(e) ^(c)), whereλ is the free-space wavelength of the input light and n_(e) ^(c) is theeffective index in the coupling region, the mode of light in the outputregion can be propagated substantially without loss or lossless.

[0061] Another way to control reflection at the output boundary is toutilize an output waveguide that is unpatterned or substantiallyunpatterned (for example, a ridge waveguide). Such an approach requiresthat the effective refractive index of the coupling region isapproximately equal to the effective refractive index of the outputregion, and particularly in the area near or at the output boundary.This approach can be implemented by gradually reducing (to zero) thevolume of each of the optical scattering elements near or at the outputboundary. In other words, as the optical scattering elements graduallyapproach the area near or at the output boundary, the volume of thoseoptical scattering elements is gradually (that is, progressively)reduced such that the volume of a given optical scattering element issufficiently less than the volume of another optical scattering elementlocated further from the output boundary (and towards or into thecoupling region). In one preferred embodiment of the present I/O ports,the structure of the coupling region is eventually the same as thestructure of the area beyond the output boundary. That area may, forexample, be the output region. The effective index of the couplingregion and that area match where the respective structures match, andsuch matching specifically occurs where the volume of the opticalscattering elements has decreased to zero (that is, when the opticalscattering elements no longer exist). For such an I/O port, the verticalwaveguide mode profile will also be matched across the output boundary.

[0062] Prior work on I/O ports has established that, in connection withcontrolling (for example, maximizing) coupling efficiency, there is arelationship between the spatial variation of the input beam intensityprofile and the spatial variation of the power loss parameter. Inparticular, and in the case of coupling regions comprising a uniformarrangement of optical scattering elements, prior work on I/O ports hasestablished that the resulting constant power loss parameter is not wellmatched to the Gaussian-like mode profile that emerges from a typicalsingle-mode optical fiber.

[0063] The present I/O ports can be further enhanced so as to furtherimprove coupling efficiency (that is, to further reduce insertion loss).In particular, optical scattering elements can be arranged in such a wayas to intentionally vary the power loss parameter within the couplingregion. Thus, another preferred embodiment of the present I/O ports isan I/O port of the type shown in FIG. 3(a). That I/O port features aGaussian-like input beam. An illustrative calculation of the optimalpower loss parameter profile, calculated using methods which are wellknown, is shown in FIG. 12, where the dashed line shows the targetGaussian beam profile, and the solid line shows the corresponding powerloss parameter profile required for relatively high coupling efficiency.The power loss parameter, which progressively increases away from theoutput boundary, is relatively small at or near the output boundary,which occurs at x=0. At or near the output boundary, the power in theI/O port, propagating towards the output waveguide, is at or near itspeak and the intensity of the Gaussian beam is relatively low. Theintensity of the input beam, which has a Gaussian profile, is relativelyhigher away from the output boundary, and it is necessary to have thepower loss parameter at an appropriate magnitude in this area in orderto achieve acceptable in-coupling of the central portion of the beam.Matching the power loss parameter in the area at or near the outputboundary is an important factor in effectively controlling insertionloss. The power loss parameter should also be well matched in the areaaway from the output boundary in order to effectively control insertionloss. Still further from the output boundary, the power loss parameterdrops off again as the Gaussian beam intensity returns to a relativelylow level. In that area, the value of the power loss parameter is not ascritical as in the other regions because both the intensity of theGaussian beam and the power in the I/O port are at relatively lowervalues.

[0064] Once a desired power loss parameter profile, effective index ofrefraction, operating frequency and coupling angle are determined, thepitch(es) and volume(s) of the optical scattering elements can then bedetermined. To determine the appropriate parameters for an opticalscattering element at a particular position in the coupling region, wemodel an infinite uniform lattice of identical optical scatteringelements using the Green's function technique mentioned earlier. Theoptical scattering element volume and pitch vary slowly and smoothlyacross the coupling region, and, as described above, the volume of eachoptical scattering element becomes relatively smaller as the opticalscattering elements approach the output boundary. An iterative designprocedure, using such a model, is used to select an appropriate pitchand optical scattering element volume at each position across thecoupling region in order to ensure that (a) the power loss parameterprofile is substantially the same as the desired profile at allpositions across the coupling region and (b) a substantially constantfrequency at which maximum coupling occurs for the given incident beamangle occurs at all positions across the coupling region. A preferredembodiment of such an I/O port comprises a layered structure having afirst intermediate layer comprising GaAs, which layer has a thickness ofapproximately 184 nm, on top of a second intermediate layer comprisingAl_(x)O_(y), and a multi-layer substrate. The multi-layer substratecomprises an upper layer, lower alternating layers and a lowest layer.The upper layer comprises Al_(x)O_(y), which layer has a thickness ofapproximately 850 nm and is located above 10 pairs of the loweralternating layers, which layers alternate between a layer comprisingGaAs and a layer comprising AlAs and have respective thicknesses ofapproximately 116 nm and approximately 133 nm. The lowest substratelayer is a relatively thick GaAs layer. The alternating layers of GaAsand AlAs serve as a reflector underneath the coupling region. Inparticular, those alternating layers enhance the branching ratio bypreventing the flow of light from the coupling region into thesubstrate. Optical scattering elements are cylindrical holes that extendthrough the intermediate layer comprising GaAs and through theintermediate layer comprising Al_(x)O_(y), and the intermediate layer ofAl_(x)O_(y) has a thickness of approximately 300 nm. The couplingregion, which subsists in the intermediate layers, covers an area ofapproximately 20 μm by approximately 20 μm. This embodiment of thepresent I/O ports is designed to effectively operate at a range offrequencies within the C-band. The input light for this embodiment ispreferably at an incidence angle θ=approximately +12°. The power lossparameter profile is chosen to match a Gaussian input beam with a modefield diameter of approximately 10 μm, with the center of that beamlocated approximately 5 μm from the output boundary. In this embodimentof the present I/O ports, the pitch is constant in the y-direction andis approximately 400 nm. The power loss parameter in this preferredembodiment is similar to that shown in FIG. 12, except that that powerloss parameter remains constant at a value of approximately 5800 cm⁻¹ atdistances from the output boundary that are less than approximately −9μm. The corresponding variation of pitch and the corresponding variationof the radius (for each optical scattering element) along the couplingregion are shown in FIG. 13 by the “+” and “x” symbols, respectively.The radius and pitch of each optical scattering element increase, in thedirection from the output boundary towards the coupling region, as thedistance from the output boundary increases. Such a relationship, whichis illustrated in FIG. 13, achieves the necessary power loss parametervariation while maintaining a substantially constant frequency at whichmaximum coupling occurs for the given incident beam angle. FDTDsimulations for this preferred embodiment show the peak couplingefficiency is approximately 86%, the center wavelength is approximately1550 nm and the FWHM bandwidth is approximately 50 nm.

[0065] In another preferred embodiment of the present I/O ports,non-uniformly placed optical scattering elements of non-uniform volumeare incorporated into an I/O port of the form shown in FIG. 3(c) toeffectively couple a Gaussian-like input beam of any polarization. Thelattice is oriented as shown in FIG. 11. The lattice pitch and opticalscattering element radii decrease as they approach the output boundary(10), and are selected such that the desired power loss parametervariation (as shown in FIG. 12) can be achieved. Such an arrangementresults in more effective coupling relative to prior I/O ports thatfeature a uniform arrangement of optical scattering elements in the sameor a similar layer structure and that couple light into a substantiallyunpatterned region. The improved coupling and low polarization dependentloss over a range of frequencies within the C-band are due to beamprofile matching and reduction of loss at the output boundary.

[0066] While examples described above have involved cylindrical orrectilinear (for example, rectangular) optical scattering elements, anincrease in coupling efficiency may be obtained with other opticalscattering elements such as elliptical holes, hemispherical holes,conical holes or angled optical scattering elements (with angled opticalscattering elements being optical scattering elements that are notnormal to the x-y plane of the I/O port), optical scattering elementscomprising more than one type of underlying (that is, foundational)optical scattering element, optical scattering elements whose underlyingoptical scattering elements are of differing volumes, and/or opticalscattering elements that are located within different layers.Furthermore, optical scattering elements characterized by reducedsymmetry (for example, rectangular holes) may be useful in certainapplications. The goal of such relatively complex optical scatteringelements is to achieve asymmetry in the unit cell in both the verticaldirection, to enhance the branching ratio, and in the horizontal (thatis, in-plane) direction, to preferentially direct light from thescattering region towards the output waveguide(s).

[0067] Another preferred embodiment of the present I/O ports is shown,in a schematic, cross-sectional view, in FIG. 14(b). That I/O portfeatures an encapsulating unpatterned superstrate layer (26) (forexample, comprising SiO₂) and optical scattering elements (27) aboveunpatterned substrate layers (29, 63). Such an I/O port may be fashionedin a two-step process. First, an intermediate layer (28) (for example,comprising silicon (Si)), as shown in FIG. 14(a), can be appropriatelypatterned with optical scattering elements. Second, an unpatternedsuperstrate layer (26), comprising, for example, SiO₂, can be attachedto the intermediate layer (28) by wafer bonding. This process allows forincorporation of a superstrate layer (that is, an unpatternedsuperstrate layer (26), as shown in FIG. 14(b)), other than air, intothe present I/O ports. The thickness of the superstrate layer (26) can,as stated earlier, be appropriately selected so as to control I/O portcharacteristics and performance. In addition, the unpatternedsuperstrate layer (26) (which, in this embodiment, is an SiO₂ layer)serves as mechanical protection by encapsulating the optical scatteringelements. Such an embodiment may provide certain commercial advantagesnot related to actual performance of the I/O port. For example,packaging of the above-described embodiment of the present I/O ports maybe facilitated insofar as packaging materials may come into contact withthe upper surface of the finished device without any concern for damageto the finished device. Further, such an embodiment may preventparticulates and other undesirable by-products of chip dicing andmanufacturing from becoming embedded in the patterned optical scatteringelements (for example, when the elements are filled with air, vacuum, ora gas). While a preferred embodiment where the encapsulation layercomprises a single SiO₂ layer has been described, a multi-layerencapsulation layer comprising, for example, Si and SiO₂, may beappropriate for certain applications of the present I/O ports.

[0068] Further improvements of the present I/O ports can be realized byappropriate design of the output region. In other words, the spot sizeof the input optical mode typically found in optical fibers can beadapted to the mode size of the optical signal in high-index-contrastoutput waveguides. For example, by etching a two-dimensional arrangementof holes into an initially unpatterned layer in which lattice spacingslowly varies transverse to the direction of propagation of lightcoupled into the waveguide, an achromatic planar waveguide lens outputregion may be formed. For example, cylindrical holes may be arrangedwith a constant lattice spacing a_(′), in the x′-direction, and aspacing between holes in the y′ direction that varies parabolically awayfrom the center line, y′=0. By appropriate design, the approximately 10μm spot coupled into the waveguide in the coupling region can be reducedin the output region to match the smaller mode profile of the outputwaveguide. Normally, the output waveguide would be chosen to beidentical to other waveguides in the rest of the planar or substantiallyplanar photonic circuit in order to facilitate connection of the outputwaveguide to the rest of the planar or substantially planar photoniccircuit in which the I/O port is used.

[0069] The present I/O ports can be used to createpolarization-insensitive photonic circuits with polarization-sensitiveintegrated photonic elements. The difficulty in achievingpolarization-insensitive operation of photonic circuits is fundamentallylinked to the axial coupling geometry illustrated in FIG. 1. In thatgeometry, incoming light of an unknown or varying polarization couplesto the transverse electric (TE) mode, which has its electric fieldparallel to the surface of the planar or substantially planar photonicdevice, to the transverse magnetic (TM) mode, which has its electricfield perpendicular to the surface of the planar or substantially planarphotonic device, or to both. Achieving polarization-insensitiveoperation of the planar or substantially planar photonic device requiresthat the layer structures and device geometries be carefully adjusted tomatch the TE and TM responses to one another, or requires incorporationof intermediate elements in the photonic circuit (with waveplates beingan example of such intermediate elements). The above-described carefuladjustments and incorporation of intermediate elements are bothdifficult to achieve in a general photonic circuit, and the need toimplement one or the other imposes restrictions on effective designs ofI/O ports and photonic circuits. With the present I/O ports, which arebased on off-normal, or normal or near-normal, incidence coupling,incoming light of different polarizations is coupled to two or moreseparate outgoing beams, each of which has the same polarization in theplanar or substantially planar device. For example, the incoming signalmay be coupled into two TE-polarized output signals propagating indifferent directions. Those two output signals can be processed by twosubstantially identical or identical optical elements, each of which isrequired to respond only to a single polarization (for example, the TEpolarization).

[0070]FIG. 15(a) illustrates a photonic circuit comprising a preferredembodiment of the present I/O ports. In that embodiment, an I/O port(30) directs two outputs of light (31, 32) of substantially identical TEpolarization to two output waveguides (33, 34), which then couple thetwo outputs of light (31, 32) into two suitable (and, in this case,substantially identical) optical elements (35, 36), the two outputs ofwhich (37, 38) are directed by two output waveguides (39, 40) to an I/Oport (41). In this balanced arrangement, PDL and PMD can be controlled(for example, reduced). Where the two optical elements (35, 36) areeffectively matched (that is, in a well-balanced photonic circuit), PDLand PMD may be effectively eliminated.

[0071]FIG. 15(b) illustrates another photonic circuit comprising anotherpreferred embodiment of the present I/O ports. In that embodiment, anI/O port (42), directs two outputs of light (43, 44) to two outputwaveguides (45, 46), which then couple the two outputs of light (43, 44)into two optical elements (47, 48), the outputs (49, 50) of which aredirected by two output waveguides (51, 52) to a two-input opticalcombiner (53), such as, for example, a multi-mode interference (MMI)combiner which is well known in the art, and then into a single outputwaveguide (54), which connects to an I/O port (55). By incorporatingsuitable optical elements (for example, variable optical delayelements), the two polarizations may be combined in-phase to yieldmaximum power in the output waveguide (54), thereby resulting in anoptical signal (62) emerging from the output port (55) with a knownstate of polarization at or just outside of the output port.

[0072] In addition to the specific materials described above (forexample, GaAs, Al_(x)O_(y), Si, and SiO₂), other materials may besuitable materials for the superstrate layer(s), intermediate layer(s)and/or substrate layer(s) of the present I/O ports, depending, forexample, on the particular application(s) at issue. For example, SiN(silicon nitride) may be a suitable material for the present I/O ports,and so too may be InP (that is, indium phosphide), an alloy comprisingInGaAs (that is, indium gallium arsenide) and an alloy comprisingInGaAsP (that is, indium gallium arsenide phosphide). Al_(x)O_(y) may beprepared by oxidation of aluminum-containing compounds such as AlAs(that is, aluminum arsenide), an alloy comprising AlGaAs, an alloycomprising InAlGaAs (that is, indium aluminum gallium arsenide) or analloy comprising InAlAs.

[0073] Many additional modifications are variations of the present I/Oports are possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the I/O portsmay be practiced otherwise than as described hereinabove.

1. A photonic input/output port, comprising: (a) a layered structurecomprising an unpatterned superstrate having at least one layer, anunpatterned substrate having at least one layer and at least oneintermediate layer sandwiched between the unpatterned superstrate andthe unpatterned substrate, (b) a coupling region that is within the atleast one intermediate layer and that comprises an arrangement of atleast one optical scattering element, and (c) at least one outputwaveguide, wherein the at least one optical scattering element has anindex contrast that is greater than or equal to approximately
 1. 2. Thephotonic input/output port of claim 1 further comprising at least oneoutput region.
 3. The photonic input/output port of claim 1 furthercomprising at least one in-plane reflector region.
 4. The photonicinput/output port of claim 1 wherein the unpatterned substrate serves asa reflector.
 5. The photonic input/output port of claim 1, wherein theat least one optical scattering element has a geometry from the groupconsisting of: cylindrical hole, elliptical hole, hemispherical hole,rectilinear trench, conical hole, angled cylindrical hole, angledhemispherical hole, angled elliptical hole, angled rectilinear trenchand angled conical hole.
 6. The photonic input/output port of claim 1,wherein the arrangement of at least one optical scattering element isuniform.
 7. The photonic input/output port of claim 1, wherein the atleast one layer of the unpatterned superstrate comprises a material fromthe group consisting of: air, GaAs, Si, SiO₂, SiN, InP, an alloycomprising InGaAs, an alloy comprising InGaAsP, an alloy comprisingAlGaAs, an alloy comprising InAlGaAs, an alloy comprising InAlAs and analuminum oxide.
 8. The photonic input/output port of claim 1, whereinthe at least one layer of the unpatterned substrate comprises a materialfrom the group consisting of: GaAs, Si, SiO₂, SiN, InP, an alloycomprising InGaAs, an alloy comprising InGaAsP, an alloy comprisingAlGaAs, an alloy comprising InAlGaAs, an alloy comprising InAlAs and analuminum oxide.
 9. The photonic input/output port of claim 1, whereinthe at least one intermediate layer comprises a material from the groupconsisting of: GaAs, Si, SiO₂, SiN, InP, an alloy comprising InGaAs, analloy comprising InGaAsP, an alloy comprising AlGaAs, an alloycomprising InAlGaAs, an alloy comprising InAlAs and an aluminum oxide.10. The photonic input/output port of claim 1, wherein the at least oneunpatterned substrate layer is located above at least one pair ofadditional substrate layers, which alternate between a layer comprisingGaAs and a layer comprising AlAs.
 11. The photonic input/output port ofclaim 1, wherein the at least one unpatterned substrate layer is locatedabove at least one pair of additional substrate layers, which alternatebetween a layer comprising GaAs and a layer comprising an alloycomprising AlGaAs.
 12. The photonic input/output port of claim 1,wherein the at least one unpatterned substrate layer is located above atleast one pair of additional substrate layers, which alternate between alayer comprising Si and a layer comprising SiO₂.
 13. The photonicinput/output port of claim 2, wherein the at least one output waveguideis a ridge waveguide.
 14. The photonic input/output port of claim 1,further comprising an output boundary, wherein the arrangement of atleast one optical scattering element comprises two or more opticalscattering elements and the volumes of the scattering materialsgradually decrease as they approach the area near or at the outputboundary.
 15. Use of the photonic input/output port of claim 1 in aphotonic circuit, with input light entering the photonic circuit atoff-normal incidence from the top plane of the photonic circuit.
 16. Useof the photonic input/output port of claim 1 in a photonic circuit, withinput light entering the photonic circuit at an incidence ofapproximately +/−12° from the top plane of the photonic circuit.
 17. Useof the photonic input/output port of claim 1 in a photonic circuit, withinput light entering the photonic circuit at an incidence ofapproximately +/−8° from the top plane of the photonic circuit.
 18. Abalanced photonic circuit, comprising: (a) a first photonic input/outputport and a second photonic input/output port, each input/output portcomprising: i. a layered structure comprising an unpatterned superstratehaving at least one layer, an unpatterned substrate having at least onelayer and at least one intermediate layer sandwiched between theunpatterned superstrate and the unpatterned substrate, and ii. acoupling region that is within the at least one intermediate layer andthat comprises an arrangement of at least one optical scatteringelement, wherein the at least one optical scattering element has anindex contrast that is greater than or equal to approximately 1, (b) afirst output waveguide, (c) a second output waveguide, (d) a thirdoutput waveguide, (e) a fourth output waveguide, (f) a first opticalelement, and (g) a second optical element, wherein the first outputwaveguide connects the first input/output port to the first opticalelement, the second output waveguide connects the first input/outputport to the second optical element, the third output waveguide connectsthe first optical element to the second input/output port, the fourthoutput waveguide connects the second optical element to the secondinput/output port and the first and second optical elements are at leastsubstantially identical.
 19. An unbalanced photonic circuit, comprising:(a) a first photonic input/output port, comprising: i. a layeredstructure comprising an unpatterned superstrate having at least onelayer, an unpatterned substrate having at least one layer and at leastone intermediate layer sandwiched between the unpatterned superstrateand the unpatterned substrate, and ii. a coupling region that is withinthe at least one intermediate layer and that comprises an arrangement ofat least one optical scattering element, wherein the at least oneoptical scattering element has an index contrast that is greater than orequal to approximately 1, (b) a second photonic input/output port,comprising: i. a layered structure comprising an unpatterned superstratehaving at least one layer, an unpatterned substrate having at least onelayer and at least one intermediate layer sandwiched between theunpatterned superstrate and the unpatterned substrate, and ii. acoupling region that is within the at least one intermediate layer andthat comprises an arrangement of at least one optical scatteringelement, wherein the at least one optical scattering element has anindex contrast that is greater than or equal to approximately 1, (c) afirst output waveguide, (d) a second output waveguide, (e) a thirdoutput waveguide, (f) a fourth output waveguide, (g) a fifth outputwaveguide, (h) a first optical element, (i) a second optical element,and (j) an optical combiner, wherein the first output waveguide connectsthe first input/output port to the first optical element, the secondoutput waveguide connects the first input/output port to the secondoptical element, the third output waveguide connects the first opticalelement to the optical combiner, the fourth output waveguide connectsthe second optical element to the optical combiner and the fifth outputwaveguide connects the optical combiner to the second input/output port.